1. Technical Field
The present invention relates to the formation of forged articles from titanium based alloys of the Ti.sub.3 Al (Alpha-2) type, and more preferably, Ti.sub.3 Al based titanium aluminides containing substantial amounts of beta (.beta.) stabilizers such as Ti.sub.3 Al+Nb alloys or Super Alpha-2 (Super .alpha..sub.2)as disclosed in U.S. Pat. Nos. 4,292,077, 4,716,020, 4,788,035 and 5,032,357 that have good elevated temperature mechanical properties, useful ductility at room temperature, and adequate sensitivity to ultrasonic inspection for detection of material defects. These titanium aluminide alloys have potential for excellent high temperature properties compared to other advanced titanium alloys, such as about 20-30% greater tensile strength at 1200.degree. F. and greater than 100.degree. F. temperature capability useful in gas turbine applications such as impellers and axial rotors.
2. Background Art
Alloys based on Ti.sub.3 Al compositions have received considerable attention for their potential use as low density, high strength, high temperature aerospace materials. However, useful application of these alloys as aerospace materials has been prevented, mainly because the alloys which have high temperature mechanical properties, do not have adequate room temperature ductility. Progress was made with Ti.sub.3 Al+Nb compositions having high .beta. stabilizers (e.g., Nb.about.10-20%, often with minor additions of Mo, V and Ta) with the purpose of increasing room temperature ductility and fracture toughness. However, property optimization and its consistency in forge-processed alloys has become difficult due to complexity involved in the thermomechanical approaches to process the material.
The titanium aluminide alloys can be forge-processed in several ways. When heated above a temperature called .beta. transus, the material essentially consists of a single phase of .beta. grains, and when cooled below .beta. transus, the alloy exhibits several phases including a dominant alpha-2 (.alpha..sub.2) phase due to transformation of the .beta. phase. The Ti.sub.3 Al based alloys, as conventionally processed, may be forged above the .beta. transus (.beta. forging) or below the .beta. transus (.alpha..sub.2 +.beta. forging). Similarly, the material after forging may be heat treated above or below the .beta. transus followed by a stabilization treatment at a lower temperature. For example, see Deluca, D. P. et al: "Fatigue and Fracture of Titanium Aluminides", Report No. WRDC-TR-89-4136, U.S. Air Force, WRDC, February, 1990 and Blackburn and Smith: "R+D on Composition And Processing of Titanium Aluminide Alloys For Turbine Engines", Report No. AFWAL-TR-82-8046, Air Force Systems Command, July 1982, and Blackburn and Smith: "Improved Toughness Alloys Based on Titanium Aluminides", Report No. WRDC-TR89-4095, Air Force Systems Command, October 1989, and Blackburn and Smith U.S. Pat. Nos. 4,716,020 and 4,292,077. One serious problem of the conventionally-processed alloys, as found in the aforementioned references, for example, is the lack of .beta. grain refinement, i.e., the .beta. grains are not recrystallized during forging, and remain very large, for example, in the aforementioned references, the .beta. grain sizes were .about.1.2 to 3.0 mm. In U.S. Pat. No. 5,281,285, less than 20% of the beta phase recrystallizes during forging.
While the large grains are acceptable with respect to elevated temperature creep properties, they are not desirable due to reduction in strength and low cycle fatigue (LCF) resistance of the forgings. Also, because of coarse grains, the forged articles made therefrom cannot be inspected efficiently. For example, when detecting internal defects by ultrasonic, non-destructive methods, the presence of large grains create "background noise" or interference which generally requires rejection of the part. The presence of small grains, however, produces sonically-quiet workpieces with minimum interference to sonic testing. In certain titanium alloy applications, such as selected aerospace applications, certain manufacturer's specification dictate that the grain size be less than 0.5 mm and preferably 0.2 mm or less. U.S. Pat. No. 5,026,520 describes a fine grain titanium alloy forging method where it is stated that grain refinement is not achieved dynamically during forging, but requires a static holding time just after forging at the forging temperature to effect static recrystallization of the coarser grains to finer sizes. For Ti.sub.3 Al based titanium aluminide alloys, U.S. Pat. No. 4,716,020 indicates that a .beta. grain size 0.15-0.2 mm is desirable but no method of producing such grain size is taught, and the mechanical properties indicated in said Patent are from forgings with .beta. grain size .about.1.5 mm.times.2.5 mm, as disclosed in AFWAL-TR82-4086 at page 20. It is generally recognized in the art of processing titanium or aluminide alloys that further .beta. grain refinement (&lt;0.1 mm) is beneficial for improved strength, ductility, LCF and ultrasonic inspection sensitivity, but no such method of refining the grains in the Ti.sub.3 Al based alloys is known to the art.
Several heat treatment approaches are possible for the forged Ti.sub.3 Al based alloys. The heat treated alloy can contain a complex microstructure having several phases depending upon the temperature and time of heat treatment. Above the .beta. transus temperature, there is only the body entered cubic phase .beta.. The .beta. transus temperature for
Super .alpha..sub.2 is about 2010.degree. F. With rapid cooling down from above the .beta. transus temperature, it is possible that no other phase, such as ordered hexagonal .alpha..sub.2, will come out and the microstructure will consist of only transformed .beta. phase, or .beta./B.sub.2. B.sub.2 is a brittle, ordered version of the .beta. phase at lower temperature. In conventional heat treatment, as has been generally employed, the cooling rate is not rapid and the .alpha..sub.2 phase comes out as platelets in a matrix of transformed .beta. grains forming what is known as Widmanstatten microstructure.
A typical heat treatment in the conventional processing, e.g., of Super .alpha..sub.2 would be solution treatment 25 to 100.degree. F. either above or below the .beta. transus, cooling at an intermediate rate in a salt bath to 1500 to 1600.degree. F. where it is held for some time before cooling to room temperature. The gradual cooling causes the .alpha..sub.2 phase to come out as platelets and the formed structure is complex containing some .beta./B.sub.2 phases. The material is stabilized at about 1200.degree. F. Typical ductility or % elongation varies from 1.5 to 3.3% at room temperature with the low ductility usually associated with a higher 0.2% yield strength at 1200.degree. F. (e.g., 80-90 ksi) and the high ductility usually associated with a lower 0.2% yield strength at 1200.degree. F. (e.g., 60-70 ksi). The ductility range however is low but is higher than that obtained if the alloy is cooled directly from the solution temperature to room temperature. In direct cooling, with the cooling rate reasonably rapid, the brittle .beta./B.sub.2 phase may dominate the structure and room temperature ductility is even lower, possible less than 1%.
The presence of coarse prior .beta. grains in the forge processed material as mentioned earlier, as well as the brittle .alpha..sub.2 platelets as the dominant microstructural constituent, as produced by the conventional heat treatment methods, result in an alloy with inadequate room temperature ductility, often low elevated temperature tensile strength and low LCF (low cycle fatigue) life. Furthermore, articles produced are not inspectable by ultrasonic methods due to the coarse .beta. grains. These are important obstacles to the use of Ti.sub.3 Al+Nb type of forged alloys in gas turbine applications.
Two deficiencies of prior art alloys are: (i) no forge processing method was known to effect dynamic recrystallization of the .beta. grains to finer grains (&lt;0.1 mm), and (ii) microstructural variations other than those dominated by .alpha..sub.2 phase were not known in order to produce improved properties. The .beta. phase shows complex phase transformation as a function of temperature and time below the .beta. transus, and in addition to .alpha..sub.2 and .beta./B.sub.2 phases, existence of an orthorhombic (O) phase has been identified. Several variants of the O phase may be present but no method to generate a stable and beneficial O phase as an important microstructural constituent was known to the art.
The present invention is directed towards resolving these deficiencies, i.e., developing forging method to refine the .beta. grains, and improving the microstructure within the .beta. grains to contain appropriate phases for higher ductility, strength and LCF (low cycle fatigue) resistance from room temperature up to critical use temperature of 1200.degree. F., and to preserving and retaining these properties following extended exposure at or below 1200.degree. F. In addition, by virtue of finer grains, the invention is concerned with improving the sensitivity of ultrasonic inspection to detect small internal defects at levels typically employed in gas turbine titanium alloy rotors.